Lactulose is generally what is prescribed in the UK to short term and long term opioid prescribed patients, for several reasons. It can be very gentle or can clear a severe blockage, depending on the dosage. It can be taken everyday, forever, without causing "tolerance". That is, it will continue to work but it will not cause the peristalsis in your colon to slow down causing lazy colon, or damage the colon's haustral folds that can make it hard to shit properly without the drugs, which is a risk with many stimulant laxatives. It both softens and lubricates faeces, drawing water into the colon, making a hard stool much easier to pass. This also helps to stimulate peristalsis by its bulking effect, counteracting the slowing effect opioids have. It isn't absorbed into the body, unlike milk of magnesia for example, which can have some other effects on the body. Lactulose can give you wind/gas and make your stomach gurgle a bit, but this usually diminishes quite quickly and usually isn't a problem with small doses. Large doses can be very unpleasant however, but this is true of a lot of laxatives.
A starting dose of 120ml is very high! The usual dose is 15ml twice a day, so you took 4 times the recommended starting dose. IMO 10ml 3 times a day is much easier in terms of side effects, you'll probably not notice any. It might take 1 or 2 days to fully work, but will be much more gentle. After that, taking 30ml 2 or 3 times a week is sufficient to maintain you without running into bloating, cramps or diarrhoea. On the days you do take it, split the 30ml into 2 x 15ml doses 12 hours apart.
Once you clear the initial blockage, you should try and incorporate Psyllium AKA Metamucil in your diet. Chances are you won't need the lactulose except on rare occassions if you take Psyllium regularly.
Back when I was abusing poppy pods hard, if I didn't take a small dose of lactulose around twice a week, I would end up not taking a shit for anywhere between 4 to 8 days. Needless to say passing such a monster shit felt like pushing out a log of concrete and took a ton of work, usually involving an enema and lubricating my ringpiece and inside of my ass with copious Vaseline. Even then it still involved much straining, pain and blood. FWIW squatting on the toilet seat makes it loads easier, I still squat, as whilst I don't need lactulose anymore, I am on methadone and I have mild constipation which can make some stools harder to pass if I were to sit "normally". I'd probably need to take lactulose if I didn't squat.
Anyway, the point of this TMI shitpost is that taken properly, lactulose and/or Psyllium can work in the background without you even being aware of it. It'll help you shit, keep you regular without noticeable side effects and spare you the pain and embarrassment of having to coax a huge solid turd out of your ass with enemas, lube and your fingers. It should be in the toolbox for any opioid user.